A variety of hormones and neurotransmitters regulate the functions in vivo through specific receptor proteins located in a cell membrane. Many of these receptor proteins mediate the transmission of intracellular signals via activation of guanine nucleotide-binding proteins (hereinafter sometimes referred to as G proteins) with which the receptor is coupled. These receptor proteins possess the common structure, i.e. seven transmembranes domains and are thus collectively referred to as G protein coupled receptors or seven-transmembrane receptors (7TMR).
G protein coupled receptor proteins exist on each functional cell surface of cells and internal organs of a living body and play very important roles as the targets of molecules, for example, hormones, neurotransmitters, physiologically active substances and the like, which molecules control, regulate or adjust the functions of cells and internal organs in the living body. These receptor proteins mediate signal transduction in a cell by binding to physiologically active substances and various reactions such as activation or inhibition of cells is caused by the thus transmitted signal.
To clarify the relation between substances which regulate complex functions in cells and internal organs of various living bodies and their specific receptor proteins, in particular, G protein coupled receptor proteins, would elucidate the functional mechanisms of cells and internal organs in various living body and thus provide a very important means for development of drugs having close relation to such functional mechanisms.
For example, in various organs of a living body, the physiological functions are controlled through regulation by many hormones, hormone-like substances, neurotransmitters or physiologically active substances. In particular, physiologically active substances present in various sites of a living body regulate its physiological functions through each of the corresponding receptor proteins. Still, there are many unknown hormones, neurotransmitters or other physiologically active substances exist in vivo and only a few receptor proteins have been reported on their structures so far. In addition, it yet remains unclear if there will be subtypes of known receptor proteins.
It is also very important for development of pharmaceuticals to clarify the relation between substances that regulate complex functions in vivo and their specific receptor proteins. Furthermore, for efficient screening of agonists and antagonists to receptor proteins in development of pharmaceuticals, it was necessary to elucidate the functions of receptor protein genes expressed in vivo and to express the genes in an appropriate expression system.
In recent years, random analysis of cDNA sequences has been actively performed as a method for analyzing genes expressed in vivo. The sequences of cDNA fragments thus obtained have been registered and published to databases as Expressed Sequence Tag (EST). However, since most EST contains only the sequence information, the function is predictable only with difficulty.
Substances that inhibit the binding of G protein coupled proteins to physiological active substances (i.e., ligands) and substances that bind to physiologically active substances thereby to induce signal transductions similar to those induced by the physiologically active substances (i.e., ligands) have been used for pharmaceuticals as antagonists or agonists specific to the receptors for regulating the biological functions. Accordingly, it is very important to discover a new G protein coupled receptor protein that is not only important for physiological expression in vivo but can be a target for developing pharmaceuticals-and to clone the genes (e.g., cDNA), in search for a specific ligand, agonist, and antagonist of the novel G protein coupled receptor.
However, not all G protein coupled receptors have been found. Even now, there are many unknown G protein coupled receptors and those for which the corresponding ligands are unidentified, that is, orphan receptors. It has thus been seriously awaited to explore a novel G protein coupled receptor and clarify its function.
G Protein coupled receptors are useful in searching for a novel physiologically active substance (i.e., ligand) using the signal transduction activity as an index and in searching for agonists and antagonists of the receptor. Even if no physiological ligand is found, agonists and antagonist of the receptor may be prepared by analyzing the physiological activity of the receptor through receptor inactivation experiments (knockout animal). Ligands, agonists, and antagonists of the receptor are expected to be used as prophylactic/therapeutic and diagnostic drugs for diseases associated with dysfunction of the G protein coupled receptor.
Hypofunction or hyperfunction of the G protein coupled receptor due to genetic variation of the receptor in vivo causes some disorders in many cases. In this case, the G protein coupled receptor may be used not only for administration of antagonists or agonists of the receptor, but also for gene therapy by transfer of the receptor gene into the body (or certain specific organs) or by transfer of the antisense nucleic acid to the receptor gene. In such a gene therapy, information on the base sequence of the receptor gene is essentially required for searching of deletion or mutation in the gene. The receptor gene is also applicable as prophylactic/therapeutic and diagnostic drugs for diseases associated with dysfunction of the receptor.
In addition, finding of an endogenous ligand to the receptor or a substance having the ligand activity enables to construct the system for screening antagonists or agonists to the receptor.